The present invention relates more particularly to a process of the type described in U.S. Pat. No. 4,692,535 to Larson et al. (hereafter, Larson), wherein a propylene oxide product suitable as an intermediate in the production of polyether polyols for high resilient flexible polyurethane foam applications is made by removing substantially all of a high molecular weight poly(propylene oxide) fraction from an otherwise commercially-acceptable propylene oxide.
As disclosed in the Larson patent, propylene oxide of an otherwise commercially-acceptable purity was found to contain a certain nonvolatile impurity (namely, polypropylene oxide) (or PPO) having a molecular weight of at least 50,000), which impurity made the propylene oxide unsuitable for making polyether polyols to be used with a polyisocyanate and blowing agent in the manufacture of acceptable high resilient flexible polyurethane foams. Polyether polyols prepared from propylene oxide having in excess of 0.1 parts per million by weight of the high molecular weight poly(propylene oxide) impurity were determined to lead to low foam rise and substantial blow hole formation in the polyurethane foams, whereas polyether polyols made from propylene oxide having reduced levels of the high molecular weight PPO impurity produced polyurethane foams with high good foam rise and without substantial blow hole formation.
The solution proposed by Larson involved filtering or percolating either crude liquid propylene oxide of 95 percent or greater propylene oxide content or propylene oxide of otherwise commercially-acceptable, 99 percent purity or better through a fixed bed of an adsorbent material at a temperature ranging from between 10° C. and 100° C. The sorbent materials suggested by Larson as suitable for this purpose are activated carbon, charcoal and attapulgite, and granular forms are said to be preferable to powdered forms of these materials. The quantities of sorbent to be used per unit volume of propylene oxide to be treated are estimated at from about 0.001 to about 0.01 grams or more of solid sorbent per gram of propylene oxide, with contact times ranging from about 1 to about 15 minutes, temperatures of from about 10 to about 100° C., and pressures ranging from atmospheric to superatmospheric.
There are various difficulties associated with the treatment of propylene oxide with activated carbon. Due to the considerable heat released upon sorption of propylene oxide on activated carbon, there are both hazards and possible damage to the carbon bed associated with excessive temperature increases during the initial or start-up phase of the activated carbon treatment process. Where the bed is contacted with liquid propylene oxide, the accompanying sorption exotherm has resulted in propylene oxide vaporization and migration in the bed which in turn causes secondary exotherms with temperatures in excess of 500° C. with extreme hazard and reactor damage. During bed changeover, a major concern has been the handling of toxic and hazardous carbon/propylene oxide/water slurries, which pose safety and environmental hazards.
Prior carbon treatment procedures have required vapour recovery systems, the provisions of steam, nitrogen, cooling, flare facilities as well as sophisticated control systems and trained operators.
Difficulties are compounded by the fact that high molecular weight poly (propylene oxide) polymer can be formed during shipping and storage of propylene oxide and this polymer formation has a pronounced adverse effect on the use of polyols formed from the propylene oxide in polyurethane foams.